In a magnetic disk device, servo data is written to a disk in order to control a location of a magnetic head on a magnetic disk. The servo data contains an error caused by distortion in a track (that is, a repeatable run-out (RRO) which is synchronous to a rotation of the disk). The magnetic disk device acquires RRO correction data (RRObit or PostCode) to correct the error caused by the RRO, and corrects a position of the head based on the acquired RRO correction data. The RRO correction data is normally written to a servo region (servo sector). Therefore, a preamble corresponding to the RRO correction data is contained in each servo region. In a case where the RRO correction data is written to the disk in the same format as user data, the preamble corresponding to the RRO correction data can be reduced in all the disks.